The present invention relates generally to an apparatus for vapor compression refrigeration systems, and more particularly, to an apparatus for water chiller refrigeration systems, a high-speed centrifugal compressor for such systems and a refrigerant for use in such systems. That is, the present invention is directed to an improved direct drive centrifugal refrigeration compressor which uses magnetic bearings to support the rotor structure and to an improved refrigerant suited to such a compressor.
A conventional centrifugal water chiller, shown schematically in FIG. 1, typically consists of the following components: an evaporator 101 (a heat exchanger which boils liquid refrigerant at a low pressure to cool circulating water), a compressor to raise the pressure of the resulting vapor, a water cooled condenser 102 (a heat exchanger which liquifies the compressed vapor at a high pressure rejecting the heat to a second circulating water loop), and an expansion device 103 which lowers the pressure of the liquid refrigerant allowing it to evaporate at a lower temperature.
The refrigerant pressure within the evaporator 101 and the condenser 102 are determined by the thermophysical properties of the particular refrigerant as well as the temperatures at which the boiling and condensation processes within the heat exchangers are designed to occur. For typical water chiller applications with water cooled condensers, the liquid refrigerant temperature within the evaporator is approximately 40.degree. F. and the liquid refrigerant temperature within the condenser is approximately 95.degree. F.
In a water chiller system, the compressor acts as a vapor pump, raising the pressure of the refrigerant from the evaporating pressure (the saturation pressure corresponding to the liquid refrigerant temperature) to the condensing pressure (the saturation pressure corresponding to the liquid refrigerant temperature). Existing compressors perform this process, specifically rotary, screw, scroll, reciprocating, and centrifugal compressors. Each compressor has advantages for various purposes in different cooling capacity ranges. For cooling capacities exceeding 140 tons, centrifugal compressors have been shown to yield the highest isentropic efficiency and therefore the highest overall thermal efficiency for the chiller refrigeration cycle. In general terms, a centrifugal refrigeration compressor consists of the following components: inlet guide vanes, one or more impellers within a housing surrounded by one or more diffusers with collectors driven by some mechanical shaft apparatus such as an engine or electric motor.
After passing over the inlet guide vanes, refrigerant vapor enters the impeller through the inlet in the compressor housing. When the impeller rotates, the refrigerant vapor is drawn axially into the passages formed on three sides by the rotating impeller hub and blades and on the fourth side by the stationary housing. The clearance between the rotating impeller and the stationary housing is made as small as practicable to minimize the leakage of vapor out of the passage. The rotation of the impeller imparts kinetic energy to the vapor which increases both the velocity and the static pressure of the refrigerant. The vapor is discharged from the impeller with significant velocity into the diffuser which lies in a radial plane perpendicular to the axis of rotation.
The vapor velocity contains both a radial component associated with the mass flow through the compressor and a tangential component imparted by the rotation of the impeller. The diffuser vanes direct the flow in aerodynamically-configured channels for highly efficient diffusion in limited space. The vapor decelerates due to the expansion of the flow area that naturally accompanies the increase in radius of the constant thickness diffuser passage. The deceleration of the flow results in the conversion of the kinetic energy of the vapor into additional static pressure rise. The vapor is discharged from the diffuser with a much lower velocity into the collector. The collector channels the fluid from the diffuser to the compressor outlet. Further deceleration of the flow due to the gradual expansion of the collector area results in additional static pressure rise. The fluid is discharged from the compressor through the outlet in the compressor housing.
In a centrifugal compressor, flow rate and pressure rise cannot be independently controlled. At a constant compressor speed, variable position inlet guide vanes allow modulation of the flow rate through the compressor. As the inlet vane angle increases, the flow rate through the compressor decreases, and the required torque and power input also decrease. The decrease in refrigerant vapor flow rate causes a decrease in the cooling capacity of the evaporator. In this way, the cooling capacity in a system such as a water chiller can be modulated to match the cooling load.
It has been widely recognized that the specific speed (a non-dimensional ratio of the flow rate to pressure rise behavior for centrifugal compressors) can be correlated to compressor isentropic efficiency and indirectly therefore to overall cooling system efficiency. High isentropic compression efficiencies have been demonstrated for compressors with specific speeds in the range from 0.8 to 1.2. The pressure ratio across a centrifugal compressor is a function of the tip speed, the product of the rotating speed and the compressor exit diameter. The flow rate through the compressor is largely a function of the inlet diameter and rotating speed. The specific speed represents, in part, a non-dimensional ratio of the inlet diameter to the outlet diameter, a factor in the compressor geometry.
The tip speed for a single stage centrifugal compressor is determined by the pressure ratio required to raise a particular refrigerant from the pressure corresponding to the evaporating refrigerant temperature to the pressure corresponding to the condensing refrigerant temperature. For conventional refrigerants operating at moderate shaft speeds (10,000 to 15,000 rpm) large diameter impellers are required to generate adequate tip speed. The large diameter impellers have large inlet areas and consequently high refrigerant flow rates and large minimum design evaporator cooling capacities. Minimum design cooling capacity is defined as the minimum cooling capacity at full load that corresponds to the smallest specific speed possible for a single stage unit at specified rotating speed and refrigerant temperature lift.
It has also been recognized that centrifugal compressors with smaller minimum capacities require higher rotating speeds. The higher rotating speed yields the desired pressure ratio for refrigeration in a small impeller with a small inlet diameter with a small flow rate and low minimum design cooling capacity. The advantage of extending the lower achievable minimum design cooling capacity is an advantage that centrifugal compressors have over other compression technologies. Shaft speeds in current centrifugal chillers have been limited to around 15,000 rpm for mechanical causes. Higher shaft speeds require tighter design and assembly tolerances, better shaft balancing, more reliable lubrication, etc. This results in much higher costs.
A typical known centrifugal compressor of the type shown schematically in FIG. 1 also contains a rotor with one or more impellers surrounded by a casing, bearings to support the rotating structure, and a prime mover such as an engine, turbine, or electric motor. Compressors can also contain speed increasing gear trains to convert low speed driver motion to high-speed impeller motion, oil pumps, oil filters, oil separators, heaters for lubricant flow, and moving seals to contain the refrigerant vapor within the casing.
The basic illustrated elements include the high speed impeller 104 mounted on a high speed shaft 105 which is supported by two or more bearings 106 of the journal type which ride on a lubricant film or of the rolling element type. The thrust load generated by unbalanced gas pressure on the impeller is absorbed by a thrust bearing 107 of the Kingsbury or tilting pad type which requires lubrication, or of the rolling element type. The high speed shaft is driven through a gear train consisting of a high speed 108 and low speed gears 109. The low speed shaft is supported on bearings and driven by a prime mover such as an electric motor 110 in this example. As a result of the use of either fluid film or rolling element bearings, an additional lubricant pump 111, lubricant filter 112 and lubricant cooler 113 are required. In machines with journal-type bearings, the pumps pressurize the lubricant before injection into the journal. In the case of rolling element bearings, the lubricant may be sprayed into the bearings in a mist.
The use of lubricant within a refrigeration compressor has several disadvantages. While providing necessary lubrication to bearings, the lubricant "contaminates" the tube wall of evaporators and condensers, thereby lowering the heat transfer coefficient, a critical thermal characteristic. To compensate for the lower heat transfer coefficient, either a large heat exchanger is required or large temperature differences for heat transfer need to be given. Increased temperature differences set a higher required temperature lift for a compressor, requiring the compressor to do more work to handle the consequences of lubrication.
The centrifugal compressor control system must also assure that the centrifugal compressor is not operated in a surge condition. Typical centrifugal compressors have a surge line that varies from machine to machine (due to minute manufacturing differences) and also varies over time due to various reasons such as machine wear or lubricant degradation of the refrigerant thermophysical properties. U.S. Pat. No. 4,608,833 to Kountz includes a learning mode which develops a dynamic surge line to account for the variation of the surge characteristics over time and between machines. Likewise U.S. Pat. No. 5,553,997 to Goshaw et. al. discusses a control system that dynamically determines the surge line of the centrifugal compressor.
Because of the disadvantages associated with the inability to generate efficient low flow rate centrifugal compressors due to low shaft speeds and for the disadvantages associated with the use of oil lubrication for centrifugal refrigeration compressors, we have recognized that there is a need for a lubricant free centrifugal compressor.
For definitional purposes, "magnetic bearings" are electromagnetic devices used for suspending a rotating body in a magnetic field without mechanical contact. The bearings can be further classified as active, i.e., requiring some type of control system to ensure stable levitation of the rotating body.
It is an object of the present invention to provide an apparatus which achieves efficient centrifugal refrigeration compression for typical operating conditions of water chiller systems and the like with a cooling capacity lower than previously obtainable. For definitional purposes, low cooling capacity centrifugal compression refrigeration systems refer to those systems with cooling capacities between 20 and 140 tons.
It is another object of the present invention to provide an improved centrifugal refrigeration compressor method and apparatus for water chiller applications.
Another object of the present invention is to provide improved minimum design cooling capacity in a refrigeration centrifugal compressor while maintaining high efficiencies over a broad stable operating range.
Another object of the present invention is to provide centrifugal compression which requires no lubrication of components, thereby reducing pumping, filtration, separation, heating, and plumbing hardware of prior art centrifugal refrigeration compressors. A resulting advantage is that removing lubricants from the compressor also reduces acids generated by chemical breakdown of the oil.
Another object of the present invention is to provide a refrigerant selection that allows compression from typical water chiller evaporator to condenser conditions in a single stage for low cooling capacity applications.
Still another object of the present invention is to provide bearings with diagnostic output of vibration, bearing forces, imbalance, etc.
Yet another object of the present invention is to provide an arrangement of components allowing the compressor to be directly driven by a high speed induction motor.
It is yet another object of the present invention to provide a control system which controls the operation of the bearings, inlet guide vane position, and motor speed to maximize compressor efficiency.
It is yet another object of the present invention to provide a centrifugal compressor whose surge points do not vary over time, thereby allowing the use of a pre-defined static surge line for the compressor.
It is yet another object of the present invention to provide an improved capacity control system of a centrifugal compressor wherein the operating point of the compressor is placed on an established operating map. That is, the map is developed during the design of the centrifugal compressor and accounts for the refrigerant thermophysical properties, temperature lift, impeller and diffuser design, and operating speed, among other things. This operating map is not changed/modified during normal operation of the compressor. Yet another advantage of the present invention is that a dynamic operating map is not necessary, thereby reducing control system complexity, and cost while increasing control system reliability.
The centrifugal compressor of the present invention functions to compress refrigerant vapor from the evaporating pressure to the condensing pressure in a centrifugal water chiller and is specifically embodied in a single stage direct drive centrifugal type compressor whose rotor structure is supported by active magnetic bearings. As distinct from other centrifugal refrigeration compressors, the compressor of the present invention has been configured such that the pressure rise developed across the compressor for standard water chiller operating conditions yields a flow rate through the compressor which results in a minimum design cooling capacity smaller than other centrifugal refrigeration compressors known in the prior art.
In addition, as distinct from other known centrifugal refrigeration compressors, the compressor of the present invention uses magnetic bearings as the primary support for the rotor structure which yields substantial operational advantages. Moreover, as distinct from other known centrifugal refrigeration compressors, the compressor of the present invention is directly driven by a high speed induction motor.
More particularly, the centrifugal compressor of the present invention includes a compressor housing consisting of an impeller housing and a diffuser housing which when bolted together enclose a single impeller. The centrifugal compressor has an inlet guide vane system to control the flow of refrigerant into the impeller housing for the purposes of modulating the cooling capacity of the water chiller to which it is attached. The impeller is mounted in a cantilever manner on the compressor rotor. An induction motor for rotating the impeller is mounted on the same shaft. On either side of the motor element, radial magnetic bearings of the type well known to those skilled in the art support the compressor rotor. An axial bearing can also be provided outside of each radial bearing. A digital magnetic bearing controller controls the operation of the bearings to maintain the compressor rotor in a stable position whether it is rotating or stationary. The induction motor speed is controlled by an inverter drive which converts the fixed 60 Hz frequency of typical electrical line power to a different frequency depending on the desired motor speed.
A currently preferred embodiment of the present invention in the form of a magnetic bearing centrifugal compressor, does not exhibit a surge line which changes or degrades over time or between like machines. Because the surge characteristic does not vary, a single surge line can be used instead of continuously calculating a "moving" surge line as the machine operates as has been done in the past. The line of the present invention does not change because the system has no lubricant which degrades over time or causes the degradation of the refrigerant's thermophysical properties. The position of the impeller relative to the stationary diffuser is maintained at a prescribed location by the magnetic bearing controller, so there is no bearing wear leading to changes in the relative position of these components. Variations in the dynamics of the rotating components over time, are automatically compensated for by electronically re-tuning the bearing during periodic maintenance checks.
The centrifugal compressor of the present invention achieves lower minimum design cooling capacities than prior centrifugal compressors have been capable of achieving in a single impeller stage by increasing shaft speed. Furthermore, the present invention allows for the removal of oil lubrication and its associated pumping, filtration, plumbing, separation and heating hardware found on conventional chillers by using magnetic bearings which have no wear and require no lubrication.
The centrifugal compressor of the current invention reduces mechanical complexity, the number of moving parts, and total compressor volume by using a high speed direct drive AC induction motor powered by a high frequency inverter drive. Of course, it will be understood that, in light of the teachings of the present invention, one skilled in this art will now be able to make changes and modifications without departing from the principles of the present invention.
Yet another aspect of the present invention involves the selection of the refrigerant used in the vapor compression refrigerant system. We have found that the consideration of minimum enthalpy rise across the compressor results in the selection of refrigerants having superior characteristics, particularly in the selection of HFC-227ea and HFC-227ca for use in water chiller systems employing a high-speed centrifugal compressor.